The son of an ironmonger and a schoolteacher, Cairncross graduated from the University of Glasgow in 1933 with a degree in German and French. He studied modern languages at the Sorbonne in Paris and then at Trinity College, Cambridge. At Cambridge he frequented left-wing circles and met other members of the future spy ring, but he did not fit in with the polished young men and pursued his future careers apart from them.

Cairncross entered the Foreign Office in 1936, having passed the entrance examinations with outstanding marks. Shortly after, he was introduced by James Klugmann, a communist from Cambridge, to a Soviet agent who invited him to aid the antifascist movement. Cairncross was transferred in 1938 to the Treasury and in 1940, after the start of World War II, to the Cabinet Office, where he became the private secretary of Sir Maurice Hankey, the chancellor of the Duchy of Lancaster. In this latter capacity Cairncross may have passed to the Soviets a copy of the MAUD report, which evaluated Britain’s prospects for building an atomic bomb.

In 1942 he was assigned as a German translator to Bletchley Park, a government research centre north of London where encrypted German military communications were decoded and disseminated to intelligence services. Cairncross smuggled many decrypted German communiqués to the Soviets, including vital messages on army movements on the Eastern Front that helped the Red Army to prepare for the Germans’ huge tank offensive at the Battle of Kursk (July–August 1943).

Cairncross transferred in 1944 to MI6, Britain’s foreign intelligence agency, where for a time he worked under Philby. In 1945 he returned to the Treasury. After the war he may have passed plans for the new NATO alliance to the Soviets. In 1951, after Maclean and Burgess had fled England to escape investigation, notes written by Cairncross were found in Burgess’s home, and Cairncross was interrogated by MI5, the British domestic security agency. He denied having spied for the Soviets, but he agreed to resign from the Civil Service.

Cairncross began a new career as a literary scholar, teaching in the United States at Northwestern University in Illinois and at Case Western Reserve University in Ohio. Over the following decades he published a number of translations and studies of the great French playwrights Racine, Corneille, and Molière, as well as a history of Christian attitudes toward polygamy, After Polygamy Was Made a Sin (1974).

In 1964, after Philby had defected to the Soviet Union, Cairncross was again interrogated by MI5, and this time he confessed to espionage. British authorities decided not to prosecute him, perhaps in exchange for receiving information from Cairncross, and both sides agreed to remain silent on his past. Cairncross continued his literary studies and writing and also worked for the United NationsFood and Agriculture Organization in Rome. In 1990 and 1995 he was named as the “fifth man” in books by two former Soviet intelligence officers. Cairncross moved back to England and prepared his memoirs, which were published after his death as The Enigma Spy (1997). Cairncross insisted to the last that he had never betrayed secrets that damaged Britain, and he was not ashamed to admit that he had given the Soviet Union information it used to win its great victory at the Battle of Kursk.

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